Targeting BCL-2 in B-cell malignancies and overcoming therapeutic resistance

Abstract

Defects in apoptosis can promote tumorigenesis and impair responses of malignant B cells to chemotherapeutics. Members of the B-cell leukemia/lymphoma-2 (BCL-2) family of proteins are key regulators of the intrinsic, mitochondrial apoptotic pathway. Overexpression of antiapoptotic BCL-2 family proteins is associated with treatment resistance and poor prognosis. Thus, inhibition of BCL-2 family proteins is a rational therapeutic option for malignancies that are dependent on antiapoptotic BCL-2 family proteins. Venetoclax (ABT-199, GDC-0199) is a highly selective BCL-2 inhibitor that represents the first approved agent of this class and is currently widely used in the treatment of chronic lymphocytic leukemia (CLL) as well as acute myeloid leukemia (AML). Despite impressive clinical activity, venetoclax monotherapy for a prolonged duration can lead to drug resistance or loss of dependence on the targeted protein. In this review, we provide an overview of the mechanism of action of BCL-2 inhibition and the role of this approach in the current treatment paradigm of B-cell malignancies. We summarize the drivers of de novo and acquired resistance to venetoclax that are closely associated with complex clonal shifts, interplay of expression and interactions of BCL-2 family members, transcriptional regulators, and metabolic modulators. We also examine how tumors initially resistant to venetoclax become responsive to it following prior therapies. Here, we summarize preclinical data providing a rationale for efficacious combination strategies of venetoclax to overcome therapeutic resistance by a targeted approach directed against alternative antiapoptotic BCL-2 family proteins (MCL-1, BCL-xL), compensatory prosurvival pathways, epigenetic modifiers, and dysregulated cellular metabolism/energetics for durable clinical remissions.

Fig. 1. Mimicking the BH3 domain to kill tumor cells.

a The BH3-mimetic venetoclax targets the antiapoptotic BCL-2 protein in sensitive cells and displaces proapoptotic BIM from BCL-2 to cause activation of effector proteins BAX or BAK leading to apoptosis via cytochrome c release. b As venetoclax does not target antiapoptotic MCL-1 and BCL-xL proteins, these proteins confer resistance via sequestration of BIM displaced from BCL-2. Epigenetic miR-377 downregulation by methylation results in increased expression of BCL-xL in resistant cells.

Fig. 2. Role and regulation of MCL-1-mediated resistance.

Transcriptional control is achieved by CDK9 associated with Cyclin T1 (CycT) to form the positive transcription elongation factor b (P-TEFb) complex. Inhibition of CDK9 reduces MCL-1 expression via blocking RNA polymerase II-mediated transcriptional elongation. Alternatively, Cyclin E/CDK2 inhibition can destabilize MCL-1 by inhibiting its phosphorylation on: (i) PEST domain residues (T92, T163) resulting in MCL-1 ubiquitination and proteasome-mediated degradation and (ii) on residue S64 that changes binding affinity to BIM, thus releasing BIM from its interaction with MCL-1 and causing BAX/BAK activation and apoptosis.

Fig. 3. Targeting a compensatory prosurvival pathway in venetoclax resistance.

The dual PI3K/mTOR inhibitor NVP-BEZ235 inhibits the PI3K and the mTOR pathway, which interferes with MCL-1 stability, thereby releasing BIM to activate BAX/BAK, leading to cytochrome c release and apoptosis. Inhibition of PI3Kδ by idelalisib and AKT by ibrutinib and MK2206 further blocks the PI3K/AKT-mediated signaling. Inhibition of these pathways reduces MCL-1 levels, leading to BIM release and apoptosis.

Fig. 4. BCL-2 family proteins: overcoming blocks in apoptosis.

Schematic illustration of the interaction between pro- and antiapoptotic proteins, and small-molecule inhibitors that impede these interactions to overcome apoptosis block. The antiapoptotic proteins are shown in the center (purple). Proapoptotic proteins that bind to all of the antiapoptotic proteins are shown on the right (green), while the proapoptotic proteins with preferential binding are shown on the left (blue). Direct inhibition by various BH3 mimetics are depicted in black text, while several indirect means to modulate the proapoptotic proteins are shown in blue text.